There is a continuous need in any industry that uses electrical appliances to properly insulate conductors. A favored type of insulation material is a mica-tape. This mica-tape may be wound around conductors to provide extremely good electrical insulation. An example of this is shown in FIG. 1. Illustrated here is a coil 13, comprising a plurality of turns of conductors 14. Each turn of the conductor 14 consists essentially of a copper bar or wire wrapped with a turn insulation 15. The turn insulation 15 is prepared preferably from a fibrous sheet or strip which may impregnated with a resin. Ground insulation for the coil is provided by wrapping one or more layers of composite mica tape 16 about the turn 14. The turn insulation is not adequate to withstand the severe voltage gradients that will be present between the conductor and ground when the coil is installed in a high-voltage generator. Therefore, ground insulation for the coil is provided by wrapping one or more layers of mica tape 16 about the turn 14. Such composite tape may be a paper or felt of laid down small mica flakes or may comprise a pliable backing sheet 18 of, for example, glass fiber cloth or polyethylene glycol terephthalate mat, having a layer of mica, usually in the form of flakes 20, bonded thereto by a liquid resinous binder. The tape may be applied half lapped, abutted or otherwise. Generally, a plurality of layers of the composite tape 16 are wrapped about the coil depending upon voltage requirements. To impart better abrasion resistance and to secure a tighter insulation, a wrapping of an outer tape 21 of a tough fibrous material, for example, glass fiber, or the like is applied to the coil.
Such a mica-tape 16 comprises a pliable backing sheet 18 of, for example, poly-ethylene glycol terephthalate or glass fabric mat, having a layer of mica flakes 20 bonded thereto the alumoxane-LCT-epoxy resin of the present invention. The tape 16 may be applied half lapped, abutted or in any other suitable manner. Generally, multiple layers of the mica tape 16 are wrapped about the coil with sixteen or more layers generally being used for high voltage coils. The number of layers may be decreased depending on the power of the generator and the effectiveness of the insulator in both its abilities to insulate electrically and conduct heat. To impart better abrasion resistance and to secure a tighter insulation, a wrapping of an outer tape 21 of a tough fibrous material, for example, glass fiber, asbestos or the like may be applied to the coil. Other types of mica-tapes will be apparent to one of ordinary skill in the art.
Mica, a group of silicates, such as KAl2 AlSi3 O10 (OH)2 (Muscovite) or KMg3 AlSi3 O10 (OH)2 (phlogopite), has long been a key component of high voltage electrical insulation in electrical machines over 7 kv, because of its particularly high dielectric strength, low dielectric loss, high resistivity, excellent thermal stability and excellent corona resistance. Presently, mica is used in the form of flakes on a glass fabric backing, which provides mechanical integrity required for machine wrapping of coils, as shown for example in U.S. Pat. Nos. 4,112,183 and 4,254,351 (Smith and Smith et al.), respectively. In many cases, mica tape is wrapped around the coil and then impregnated with low viscosity liquid insulation resin by vacuum-pressure impregnation (“VPI”). That process consists of evacuating a chamber containing the coil in order to remove air and moisture trapped in the mica tape, then introducing the insulation resin under pressure to impregnate the mica tape completely with resin thus eliminating voids, producing resinous insulation in a mica matrix. This resin is subsequently cured by a prolonged heating cycle.
Problems, however, arise when the mica-tape becomes damaged through tears or dents. When the tape is damaged the dielectric strength in that area is lost or reduced. Even an area of minor damage will allow for a discharge of electricity, which will cause further breakdown in the insulation in and around that damaged point. FIG. 2 illustrates a cross sectional view of a damaged area on a mica-tape insulator. This simplified view shows a layer of mica-tape 16 atop a conductor 14. The mica-tape is shown as layers of mica 24 interspersed with glass fibers 26. At the point of damage 28, the mica layers and glass fibers are disrupted. Lesser damage, such as a dent, might result in the compacting of layers with only minimal amounts of fraying, while more serious damage, as shown here, may result in a complete cut through the entire tape.
Typically, when the mica-tape becomes damaged it needs to be replaced. This involves a time-intensive and costly replacement of the entire mica-tape on that conductor. Because the replacement of the entire mica-tape is such an onerous task, efforts have been made to come up with a way of fixing the tape rather than replacing it.
One such method includes applying a patching resin to the damaged area. This resin is typically the same types of resin used to originally impregnate the mica-tape, as discussed above. The patching resin contains mica particles, and when cured the effort is for the resin patch to mimic the mica-tape structure. FIG. 3 shows a example of a repaired area using a patching resin. The cured resin 30 containing the mica particles 32 fills in the damaged area on the mica-tape 16. Although the resin itself penetrates between the damaged layers, the mica particles do not due to their size. This essentially creates a heterogeneous patch, where there is a physical interface between the mica of the mica-tape and the mica of the patching resin. This creates a weak spot at the edges of the patched area 33 that has a reduced dielectric strength and will discharge electricity. This electrical discharge will then cause further breakdown of the mica-tape and patch.
Therefore the patching resin has limited and often only temporary benefit in repairing damaged mica-tape, and often the mica-tape still needs to be replaced. What is needed is an improved patching resin that will provide sufficient dielectric strength to the repaired area and not allow for any weak spots.